1. Field of the Invention
The invention relates generally to interferometry and more particularly to systems and methods for generating multiple triggers for each cycle of a sampling signal in an optical interrogator.
2. Related Art
The commercial success of optical fiber telecommunications has fostered the growth of optical fiber sensing applications by providing a ready supply of low cost, high quality components and test equipment. Another enabling technology for fiber sensing was the discovery of the ultraviolet (UV) photosensitivity in optical fiber. Photosensitivity allows the alteration of the internal structure of a fiber waveguide. Modification of the waveguide can be employed for a number of useful purposes, one of which is to induce a periodic modulation of the refractive index along the fiber core to create a wavelength selective reflector called a fiber Bragg grating (FBG). FBG's can be produced conveniently and very inexpensively during the fiber draw process. The period of the modulation of the refractive index determines the wavelength reflected by the FBG. After the FBG is formed, the grating period of the FBG can be physically altered by changing the mechanical load on the fiber, or by changing its temperature. By monitoring the wavelength reflected from a FBG, the FBG can be used as a transducer for both strain and temperature.
An important recent development in the use of FBGs is the use of optical frequency domain reflectometry (OFDR), which is a sensing technique that can be used to monitor FBGs or other sensors. This technique can be used to interrogate hundreds or thousands of FBG's distributed along the length of a single optical fiber. The OFDR technique has many applications where light weight, immunity to electromagnetic interference, high sensor density, and remote readout are important considerations. These applications include monitoring sensors such as FBGs, providing diagnostics on optical fiber networks and cables, including the intrinsic Rayleigh scatter of the optical fiber, monitoring the condition of aerospace structures, monitoring industrial processes, and monitoring sub-marine and oil well systems.
One of the problems with conventional OFDR systems of the type described above is that the sinusoidal reference signal provides sampling triggers only once per period, for instance at rising zero-crossings. When a reference interferometer is used for sampling, the frequency of the sampling limits the frequency of signals from the device under test that can be resolved by the system (because of the Nyquist criteria), which in turn limits the length of the device under test. If it is desired to increase the sampling frequency and thereby increase the possible length of the device under test, the path length difference of the reference interferometer must be increased. This may present practical difficulties, however. It would therefore be desirable to provide systems and methods for increasing the rate of the sampling signal without increasing the length of the reference interferometer.